This invention relates to a catalyst and process for the direct oxidation of olefins to epoxides by molecular oxygen.
The direct oxidation of olefins to epoxides by molecular oxygen is well-known. The usual catalyst contains metallic or ionic silver, optionally modified with promoters and activators, such as alkaline earth metals or metal salts. U.S. Pat. No. 4,007,135, for example, lists many promoters, including calcium, magnesium, strontium, cadmium, and copper. Most of the catalysts contain a porous, inert support or carrier, upon which the silver and promoter are deposited. A review of the direct oxidation of ethylene in the presence of such silver catalysts is given by W. M. H. Sachtler, C. Backx, and R. A. Van Santen in Catalysis Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
Some patents of the prior art teach the use of catalysts containing silver and silicon in the direct oxidation of olefins. Specifically, U.S. Pat. No. 3,585,217 teaches a catalyst of silver silicate, optionally containing activator compounds, for use in the epoxidation of ethylene. This catalyst is disclosed to have a low activity. U.S. Pat. No. 4,272,443 teaches a catalyst comprising a support coated with lithium silicates and silver salts, such as silver silicate, for the epoxidation of ethylene. Japanese Koho 1978 - 39,404 (Oda) teaches a catalyst containing predominately elemental silver, and lesser amounts of cadmium and silicon oxides for the epoxidation of propylene.
It is well-known that most catalysts for the direct oxidation of ethylene to ethylene oxide do not give comparable results in the corresponding oxidation of the higher olefins, such as propylene. It would be desirable to have a catalyst for the direct oxidation of olefins which achieves a high selectivity for olefin oxides, particularly the higher olefin oxides. It would be even more desirable if such a catalyst were to possess a high level of activity.